The present invention relates to a moisture detector and particularly to a moisture detector adapted to detect moisture in plywood veneer with a minimum of apparatus, expense and complexity.
Some form of moisture detector is frequently employed at the outlet of a plywood veneer dryer so that wet pieces of veneer can be identified and redried. Many such devices employ conductive circuits, for example utilizing wire brushes which contact the veneer and register electrical conduction produced by moisture. Unfortunately, the wire brushes are subject to breakage and shorting whereby the moisture indications tend to become inaccurate. A more advantageous form of moisture detector is the non-contacting variety, for example employing capacitive plates spaced from the veneer which indicate moisture by coupling an A.C. signal between the capacitive plates and the veneer when the veneer is sufficiently wet. Some non-contacting devices are not particularly sensitive. A desirable configuration employs a transmitting plate adjacent the veneer, and a receiving plate offset along the path of the veneer for receiving an A.C. signal under moisture conditions. This type of moisture detector is exemplified by my U.S. Pat. No. 3,523,243, granted Aug. 4, 1970.
In a typical veneer handling installation, the strips or sheets of veneer are conveyed on a conveyor, e.g. including steel rollers which are ordinarily grounded to equipment ground and earth ground. When the veneer is extremely wet, the A.C. signal merely becomes shorted to ground via the conveyor rollers. Of course, a conveying system could be used which is essentially non-conductive, e.g. employing insulated rollers or the like, but this kind of insulation would involve extra expense. It might be thought a moisture detection system could be employed which is completely isolated electrically from its surroundings and therefore not subject to being grounded by the conveying equipment. However, an ungrounded system results in undesired interference and cross talk between adjacent detectors, especially when veneer is wet and connection is effectively made through conveyor ground via the wet veneer at an uncontrolled level.